The present invention, in some embodiments thereof, relates to the field of chemistry, and more particularly, but not exclusively, to novel processes for oxidation of alcohols to aldehydes, to noble metal-polyoxometalate complex catalysts (NM-POM) such as palladium(II)-polyoxometalate complexes (Pd-POM) for use in these processes and to methods of preparing these complexes.
The catalytic oxidation of alcohols using ground state molecular oxygen, O2, as a primary oxidant, has concentrated many efforts in recent years, and attracted many researchers expressly motivated by the apparent ecological advantage that such a transformation would have, compared to other methods.
There are three general rules of thumb that may be used when describing the reactivity and selectivity in these oxidation reactions: i) benzylic alcohols are the most reactive substrates, allylic alcohols have intermediate reactivity and saturated aliphatic alcohols are the least reactive; ii) typically secondary alcohols are more reactive than primary alcohols; and iii) while oxidation of secondary alcohols is typically selective to ketones, the selective oxidation primary (non-benzylic) alcohols to aldehydes is more difficult and over-oxidation to carboxylic acids is not uncommon.
In the context of selective oxidation of primary aliphatic alcohols to the corresponding aldehydes, there are some notable examples of catalysts that are selective for this transformation, which also clearly show specificity for primary versus secondary alcohols. Some of the most studied systems are those that combine metal catalysts with nitroxyl radicals, typically 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), as co-catalysts. In these catalytic systems an in situ formed nitrosonium cation oxidizes alcohols to aldehydes and the hydroxylamine formed is re-oxidized by O2 in the presence of an additional metal catalyst. Other examples include a CuII-iminosemiquinone catalyst that oxidizes primary alcohols to aldehydes with formation of H2O2 and with which secondary alcohols do not react, co-catalytic systems comprised of Ru(PPh3)3Cl2/hydroquinone, water-soluble diruthenium complexes, nitrosylruthenium-salen catalysts with photo-irradiation, Os—Cr and Ru—Cr heterobimetallic complexes, and Au—Pd supported on TiO2, although the results obtained for the latter are equivocal since 3-octanol was as reactive as 1-octanol, but 2-octanol was not reactive.
A very significant portion of the research in the area of aerobic oxidation of alcohols has been directed at the use of noble metals, and in particular at the use of palladium (Pd) complexes as well as Pd colloids as catalyst in homogeneous and heterogeneous reaction systems.
A proposed mechanism of palladium(II)-catalyzed oxidations of alcohols in the presence of O2 is illustrated in Scheme 1 below, wherein B denotes a base, L denotes a metal-complexing ligand and R and R′ denote, for example, different alkyl residues.

The mechanism presented in Scheme 1 above, is generally thought to involve (a) the ligation of the alcohol by reaction of the conjugate alkoxide with the transition metal, followed by (b) β-elimination to yield the carbonyl (ketone or aldehyde) product and a two electron reduced Pd(0) and/or Pd(II)-H complex. The rate determining step is often (c) the O2 dependent re-oxidation step, and involves the intermediate formation of H2O2, which is typically decomposed by disproportionation. Notably, basic conditions are beneficial for formation of alkoxides and therefore to initiate the catalytic cycle.
In general, palladium-based oxidation catalysts suffer from instability and rapidly degrading aptitude for regeneration for repetitive use, and in addition are expensive. Attempts to stabilize palladium catalysts included fixation to polymers (also referred to as Pd-support) and complexation with various ligands. These attempts were also directed at achieving some degree of selectivity and control in the oxidation reaction with respect to the reacting species and the end products of these reactions. However, although there are isolated examples of oxidation of aliphatic primary alcohols to corresponding aldehydes (see, for example, Nishimura, T. et al., Tetrahedron Lett. 1998, 39, 6011-6014; Nishimura, T. et al., J. Org. Chem. 1999, 64, 6750-6755; Jensen, D. R. et al., Angew. Chem. Int. Ed. 2003, 42, 3810-3813; Choudary, D. et al., Green Chem. 2006, 8, 479-482; and Pillai, U. R. et al., Green Chem. 2004, 6, 161-165), there are no reports hitherto which describe selective oxidation of primary over secondary aliphatic alcohols coupled with selective formation of aldehydes, afforded by catalytic oxidation using palladium-based catalysts.
U.S. Pat. Nos. 6,229,028, 6,573,394 and 6,762,310 teach processes for the catalytic epoxidation of alkenes using transition metal (e.g., ruthenium, nickel, cobalt and molybdenum) substituted polyoxofluorometalate complexes in the presence of molecular oxygen.
Palladium (Pd) polyoxometalate complexes such as the compounds Q12{[WZnPd2(H2O)2](ZnW9O34)2} wherein Q is Na or K and the compound Q12{[WZnPd2(H2O)2](ZnW9O34)2} wherein Q is a quaternary ammonium salt were disclosed by Tourné, C. M. et al. in J. Chem. Soc., Dalton Trans. 1991, 143-155; and in Neumann, R., et al., Inorganic Chemistry, 1995, 34, 5753-5360, respectively.